The present invention relates generally to network interfacing, and more particularly, to a frame generation circuit in a transmitter operating in a frame switched multiple access network.
Networks serve the purpose of coupling data between many remotely spaced computing devices, such as computers, file servers, printers etc., so that valuable computing resources can be shared amongst the various devices.
A commonly known technique for transmitting data across a network is to break the data file into smaller frames, each of which is individually modulated onto the carrier and transmitted to the destination. At the destination, the carrier is demodulated to recover the data and the frames are sequenced and the data file is recovered.
Each frame includes a portion of the data file along with overhead data for routing the frame to the destination device. When such technique is used in a network, the network is commonly known as a frame-switched network or packet-switched because each frame, or packet, can be routed to a different destination across a multiple access topology.
In the absence of any distortion of the carrier signal across the network medium, the received carrier would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated without error using known mixing techniques, the digital data could be recovered using known sampling algorithms, and the data file can be readily recovered by simply re-sequencing the frames.
However, the network topology tends to distort the high frequency carrier signal. In a multiple access cable network, the distortions are typically due to reflections of the transmitted carrier caused by numerous branch connections and different lengths of such branches. Such problems are even more apparent in a network which uses home telephone wiring cables as the network cable medium because the numerous branches and connections are typically designed for transmission of plain old telephone system POTS signals in the 0.3-3.4 kilohertz frequency range and are not designed for transmission of high frequency carrier signals on the order of 7 Megahertz.
A typical approach for recovering transmitted data frames at a receiver operating in such an environment includes the use of an adaptive equalizer for filtering noise and distortion on the received carrier signal. In theory, an equalized signal should match the signal originally transmitted such that a slicer can accurately map the signal to defined constellation points to recover the originally transmitted data.
To operate an adaptive equalizer, each frame includes a training sequence as part of its overhead. The training sequence is a sequence of pre-defined data bits and, during receipt of the training sequence at the receiver, various equalizer coefficients can be calculated and tested to determine the correct combination of equalizer coefficients. Because the pre-defined data bit sequence is known, the equalized signal can be readily compared to the predefined data bits to determined optimal coefficients for receipt for the frame.
The predominant method of generating and transmitting each frame in a frame switched multiple access network includes 1) parsing the data file into segments consistent with the desired frame size, 2) adding the overhead to each data segment including frame sequencing, destination routing/addressing, error correction, and training sequence; 3) encoding the frame into a low frequency base data signal; and 4) modulating the base data signal onto a high frequency carrier signal. The high frequency carrier signal is then transmitted across a network medium, via differential voltage, RF signal, modulated illumination, or other physical layer modulation scheme to a remote computing station. At the remote computing station, the high frequency carrier signal must be received and demodulated to recover the original base data signal.
Typically the transmitter includes a processor which executes code for parsing the data file into frames and generating the overhead bits and training sequence bits. A physical layer circuit then modulates the data onto a carrier which, in the case of a QAM modulation scheme includes mapping the data to defined constellation points to generate a base band data signal and mixing the base band data signal up to carrier frequency.
Processor based circuits for performing such frame generation functions are relatively expensive and can consume substantial power. This requires high-speed and costly digital signal processing circuits. Such circuits consume substantial of power and are relatively expensive. As a result, such circuits are not practical in battery powered devices for power consumption reasons, and are unsuitable for inexpensive consumer network devices such as smoke detectors, door openers and other devices requiring inexpensive network access.
Therefore, based on recognized industry goals for size, cost, and power reductions, what is needed is a device and method for frame generation which does not suffer the disadvantages of known systems.
A first aspect of the present invention is to provide a network transmitter for generating data frames for transmission on a network medium. The transmitter includes a frame generation circuit which comprises a register for storing a predefined bit sequence corresponding to a network standard training bit sequence.
A first multiplexer includes a first input coupled to the register for receiving the predefined bit sequence and a second input for receiving bits representing the data for transmission. The first multiplexer is controlled by a select signal and generates an output in accordance with the select signal.
A schedule circuit drive the select signal to provide for the first multiplexer to select bits from the register during a portion of the frame corresponding to a frame training sequence and to select bits representing the data during a portion of the frame corresponding to a data portion.
Further, the schedule circuit may drive the select signal to cause the first multiplexer to select bits from the register during a portion of the frame corresponding to an end of frame sequence.
The network transmitter may further include a media access controller which receives a data file for transmission from an upper layer application and makes the bits representing the data available to the frame generation circuit on a data bus. The data bus may be a 4-bit bus and the frame generation circuit may include a serializer register for converting the 4-bit nibble into serial data.
The frame generation circuit may further include a scrambler which receives the bits representing data and mixes the bits into a scrambled format to provide for adequate bit transitions to limit electromagnetic noise outside a predefined transmission band. A second multiplexer may select between unscrambled bits and scrambled format bits in accordance with a bypass signal driven by the schedule circuit. The bypass signal provides for the selection of unscrambled bits during a first unscrambled portion of the frame and scrambled format bits during a scrambled portion of the frame.
The output of the first multiplexer may be coupled to a mapper which generates a baseband data signal in accordance with an encoding constellation. The mapper may operate at multiple payload encoding rates. As such, the schedule circuit may provide a signal to the media access controller indicating a data rate at which the media access controller may provide data bits to the frame generation circuit to correspond to the payload encoding rate.
Further, the mapper may operate at multiple baud rates and the signal to the media access controller may provide for a data rate which corresponds to the payload encoding rate and the baud rate.
A second aspect of the present invention is to provide a method of generating frames in a network transmitter. The method comprises: a) making data to be transmitted available on a data bus; b) storing a predefined bit sequence in a register corresponding to a network standard training bit sequence; c) selecting between the predefined bit sequence and the data for transmission in accordance with a select signal to generate output data; and d) generating the select signal to select bits from the register during a portion of the frame corresponding to a frame training sequence and generating the select signal to select the data for transmission during a portion of the frame corresponding to a data portion. Further, the select signal may be generated to select bits from the register during a portion of the frame corresponding to an end of frame sequence.
The method may further include receiving a data file for transmission from an upper layer application and making data available to the frame generation circuit on the data bus. The data bus may be a 4-bit data bus and the method may include serializing the 4-bit nibbles to generate serial data.
Further yet, the method may include scrambling the serial data to provide scrambled format data which includes an adequate frequency of bit transitions limit electromagnetic noise outside a predefined transmission band. The data may be selectively scrambled in accordance with a bypass signal. The bypass signal may be generated to provide for selecting scrambled data bits during a scrambled portion of the frame and selecting data bits from the media access controller during an unscrambled portion of the frame.
A mapping step may be included to map the output data to predefined constellation coordinates to generate a base band data signal. The mapping step may include selecting between at least two predefined constellations, each representing a separate payload encoding. As such, the method further providing a signal to the media access controller indicating a data rate at which the media access controller may provide data, the data rate corresponding to the payload encoding rate. Further, the mapping step may include selecting between at least two baud rates and the signal to the media access controller may provide a data rate corresponding to both the payload encoding rate and the baud rate.